Stretched tubular film for holding foodstuff

ABSTRACT

A tubular film for encasing food. The tubular film comprises a material which has been regenerated from a viscose solution. The tubular material is longitudinally stretched by from +8 to +40 percent and transversely stretched by from -20 to +40 percent while wet and the stretch is maintained during drying, said transverse stretch and longitudinal stretch being sufficient to obtain a surface area increase of at least 10 percent from the point of regeneration. In general, the tubular material has a wall thickness of from about 60 to about 90 percent of the same tubular material except without the combined stretch. The invention also includes the method for making the tubular material by stretching the regenerated tubular material while wet and maintaining the stretch during drying.

This is a continuation-in-part of patent application Ser. No. 08/568,076 filed Dec. 6, 1995 and a continuation-in-part of patent application Ser. No. 08/797,766, filed Feb. 7, 1997, now U.S. Pat. No. 5,952,064.

BACKGROUND OF THE INVENTION

This invention relates to tubular film for encasing food products such as sausage and cheese. The invention more particularly relates to such casing material made from regenerated cellulose.

Food casings made from regenerated cellulose from xanthate viscose have been known for an extended period of time and can be made using methods well known to those skilled in the art. In particular, xanthate viscose is made by treating cellulose from cotton, wood, or other plant fiber with alkali followed by dissolving in carbon disulfide. The resulting viscose is then extruded through a die to form a tube. The resulting tube is then regenerated, usually by passing it through a coagulating bath containing coagulating materials such as sodium sulfate, and sulfuric acid. The casing is then dried.

U.S. Pat. Nos. 2,999,756 and 3,835,113 describe such processes in detail.

It is also known that cellulose containing casings can be made by extruding viscose onto a cellulose fiber web, usually a non-woven paper, shaped in the form of a tube, followed by regeneration. Such casings are known as “fibrous” casings.

It has been recently discovered that cellulose casings can also be made by precipitation (regeneration) from solutions of cellulose in a mixture of amine oxide and water. N-methyl morpholine amine oxide (NMMO) being a preferred amine oxide for this purpose.

The term “regenerated cellulose food casings” as used herein is intended to include both of the xanthate viscose and amine oxide types of casings.

Regenerated cellulose food casings have been successful for a number of reasons including relatively low cost, permeability to smoke, strength and dimensional stability.

Despite the advantages of regenerated cellulose food casings, there are a number of areas where properties could be improved. In particular, longitudinal dimensional stability is not as good as desired, especially when the casing is wet, i.e. a stuffed food casing will have unacceptable longitudinal stretch when hung vertically, e.g. in a smoke house or for purposes of curing. Such stretch can be as much as ten percent or more.

While regenerated cellulose food casings have fair permeability, it is still not as good as desired. And in order to obtain sufficient strength, the wall of the casing has been thicker than desired. Thicker walls use more viscose material than desired, decrease permeability, and reduce the quantity (unshirred length) of casing that can be shirred (longitudinally folded and compressed) into a standard unit of shirred length.

Attempts have been made to make such casings with thinner walls by variation of processing conditions, e.g. chemical changes in the regenerating bath as described in U.S. Pat. No. 4,590,107. Unfortunately, such thin wall casings have had insufficient burst strength and tensile strength for commercial application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a tubular film of the invention.

FIG. 2 shows a block diagram of the method of the invention.

BRIEF DESCRIPTION OF THE INVENTION

The invention comprises a tubular film which may be used for encasing food. The tubular film comprises a material which has been regenerated from a solution of cellulose in a solvent which usually, but not always, comprises either carbon disulfide or a mixture of amine oxide and water.

More particularly, the invention comprises a tubular film comprising a material which has been regenerated from a solution of cellulose and dried. The tubular film, after regeneration, is longitudinally stretched by from 8, preferably from about 10, to about 40 percent and simultaneously transversely stretched by from −20 to +20 percent while wet. The combined longitudinal and transverse stretch is sufficient to increase the surface area of the film by at least ten percent from the surface area immediately after regeneration. The stretch is maintained during drying. The resulting tubular film has a wall thickness of from about 60 to about 90 percent of a tubular film which is identical except without the combined stretch.

The total transverse stretch from extrusion is usually less than 75 and preferably less than 50 percent of the longitudinal stretch after regeneration, to obtain a longitudinal crystalline orientation.

In other words, the tubular film of the invention has a wall thickness of from about 60 to about 90 percent of a prior art tubular film which would be identical, except for being unstretched as described herein. Such a prior art film is not stretched after regeneration to obtain at least a 10 percent increase in surface area and such a prior art film is not longitudinally stretched after regeneration by more than 8 percent. Further such a stretch is not maintained in any prior art film while the film is dried. As a result, such a prior art film does not have the unique properties of the film of the invention, i.e. reduced film thickness, high burst strength, better permeability, superior tensile strength, good dimensional stability and a rewet longitudinal shrinkage of at least 2 and as much as 7 percent.

The invention also includes the method or process for making the above film.

In particular the method comprises:

extruding a tube from viscose;

coagulating the tube in a bath comprising acid and salt to form a regenerated cellulose tubular film;

longitudinally stretching the film by from about 8 to about 40 percent and simultaneously transversely stretching the film by from −20 to +20 percent while wet, the combined longitudinal and transverse stretch being sufficient to increase the surface area of the film by at least ten percent from the surface area immediately after regeneration; and

maintaining said combined stretch during drying, to form a dry tubular film having a wall thickness of from 60 to 90 percent of the same tubular film without the combined stretch.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the invention, the tubular material 10 comprises regenerated cellulose 11. The tubular film has many uses as a packaging and wrapping material. The tube may, for example, be split to form a film having essentially the same uses as cellophane film. The tubular material is especially suitable for use as a food casing for foodstuffs such as sausage and cheese.

The casing is made by extruding viscose which may be any modified or unmodified cellulose solution, e.g. a solution of cellulose in an amine oxide or in a mixture of copper hydroxide and ammonium hydroxide (cuprammonium process) or a solution of cellulose aminomethanate. However, the viscose is usually alkali cellulose reacted with carbon disulfide (xanthate viscose) or a solution of a non-derivatized cellulose in amine oxide and water. The viscose is extruded either into the form of a tube or to impregnate a fiber web formed in the shape of a tube. The tube is preferably unreinforced so that the reinforcement does not restrict the stretching operation.

The viscose is then regenerated. Such regeneration primarily occurs in one or more acid and salt baths in the case of xanthate viscose. Regeneration as used herein means the essentially complete regeneration obtained in a final regeneration bath before drying. In the case of derivatized cellulose, where regeneration occurs by removal of solubilizing groups, it is believed that such regeneration results in the removal of at least 90 percent of solubilizing groups added to the cellulose during the process (e.g., CS₂ in xanthate viscose, and aminomethanate groups in aminomethanate viscose). The regenerated film before drying is self supporting and stretchable. In solvent processes which dissolve unmodified cellulose, e.g., solutions in an amine oxide, regeneration means the final solvent removal step to form a self supporting stretchable film, before drying.

While it is still wet, after regeneration, the tubular material is longitudinally stretched by from 8 to 40 percent, and usually from 10 to 20 percent, and transversely stretched by from −20 to +40 percent, and usually 0 to 10 percent. The combined stretch is sufficient to provide a net increase in surface area of at least +10 percent and preferably at least +15 percent after the point of regeneration. Transverse stretch, after regeneration, is usually less than 75% and preferably less than 50% of longitudinal stretch after regeneration. The stretch is maintained during drying. In general, the tubular material has a wall thickness 12 of from about 60 to about 90 percent of a tubular material which is identical except without the combined stretch after regeneration maintained during drying.

In general, for a food casing of a diameter usual for a hot dog, the casing has a wall thickness of between 14 and 25 microns (micrometers) and usually between 21 and 23 microns.

In addition the food casing of the invention has a permeability to smoke and other vapors which is from about 20 to about 50 percent or more greater than the same casing except not being stretched in accordance with the invention.

The other physical properties of the casing of the invention, despite having a significantly thinner wall, are surprisingly close to unstretched casing and often even better. For example the burst pressure is at least 88 percent, and usually at least 95 percent of the burst pressure of a casing unstretched in accordance with the invention. The longitudinal tensile strength is at least 85 percent, usually at least 90 percent and commonly over 100 percent of the longitudinal tensile strength of the same casing except for being unstretched as described herein.

A unique aspect of the casing of the invention is that it has a rewet shrinkage along the longitudinal axis of at least 2 and usually at least 3 percent. The rewet length is usually −2 to −7, and more often −3 to −6 percent from the dry casing.

The following examples serve to illustrate and not limit the present invention.

EXAMPLE 1

In general the tubular film of the invention was made by the following steps shown in the process blocks of FIG. 2:

1. extruding a tube from viscose;

2. coagulating the tube in a bath comprising acid and salt to form a regenerated cellulose tubular film;

3. longitudinally stretching the film by from +8 to +40 percent and transversely stretching the film by from −20 to +20 percent while wet, said transverse stretch being less than 75 percent of the longitudinal stretch, the combined longitudinal and transverse stretch being sufficient to increase the surface area of the film by at least 10 percent from the point of regeneration; and

4. maintaining said stretch during drying, to form a dry tubular film having a wall thickness of from about 60 to about 90 percent of a tubular film which is identical except for having a combined longitudinal and transverse stretch insufficient to increase surface area by at least 10 percent and except for being longitudinally stretched by less than 8 percent.

Viscose was made essentially as described in U.S. Pat. No. 3,835,113 and, except as otherwise indicated, was extruded and regenerated substantially as described in U.S. Pat. No. 2,999,756. The die was for a code 24 casing (about 25.4 mm wet flat stock width). The take up speed after drying was varied by from −2.5 to +30 percent from the speed through the coagulation (regeneration) bath. The coagulation (regeneration) bath contained a sulfuric acid concentration of 132 grams per liter and a sodium sulfate salt concentration of 253 grams per liter. For purposes of plasticization, the casing was passed twice through tanks containing glycerine to obtain a glycerine content of about 15.4 percent. The flow of viscose at the point of extrusion is sufficient to yield a finished film thickness of about one mil if unstretched. After extrusion, the casing is inflated with air to expand the diameter to about 20 mm.

The results are shown in Table 1. Table 1 clearly shows the effects of maintaining longitudinal stretch in the dryer. Strength properties are not significantly altered but the thickness of the casing is substantially less and longitudinal stretch after drying is greatly reduced. Permeability studies show better permeability at the reduced thickness.

In Table 1., the following abbreviations are used:

BDG=bone dry gauge, the thickness of the dry film in grams of cellulose per ten meters of length.

DFW=dry flat width., 0.24 means code 24 as used in the industry which is a width of about 32.7 mm.

gpm=gallons per minute.

EXAMPLES 2-6

The procedure of Example 1 was followed except that various longitudinal and transverse stretches were imparted to the regenerated tubular film by varying take up speeds at the end of the film forming line to vary longitudinal stretch and by varying air inflation to vary transverse stretch. The results are shown in Table 2. These results clearly show that comparable burst pressures can be obtained with more than 40 percent less film thickness (60% combined stretch which increases surface area by 68%).

EXAMPLE 7

In general, the tubular film of the invention when precipitated from an amine oxide (e.g., NMMO) water solution is made by the following steps, as shown in the process blocks of FIG. 2:

1. extruding a tube from an NMMO water solution;

2. precipitating the tube in an aqueous bath to form a wet cellulose tubular film;

3. longitudinally stretching the film by from 8 to 30 percent after precipitation and transversely stretching the film by a total of from −3 to +10 percent from extrusion, the total transverse stretch from extrusion being less than 75 percent of the longitudinal stretch after precipitation; and

4. maintaining said stretch during drying, to form a dry tubular film having a wall thickness of from 75 to 90 percent of a tubular film which is identical except for being longitudinally unstretched.

More specifically, in a particular example, an NMMO water solution of cellulose made essentially as described in U.S. Pat. Nos. 4,246,221; 4,416.698 and 5,277,857, except as otherwise indicated, is extruded and regenerated substantially as described in U.S. Pat. No. 5,277,857 and WO 9535340. The die is for a code 24 casing (about 25.4 mm wet flat stock width). The extrusion speed is set at about 28 feet per minute. The speed at the end of the dryer is varied from about 27 to about 37 feet per minute to vary the stretch from −2.5 percent to +30 percent after precipitation. For purposes of plasticization, the casing is passed twice through tanks containing glycerine at the wet end to obtain a glycerine content of about 15.4 percent. The flow of cellulose solution at the point of extrusion is sufficient to yield a thickness of about one mil if unstretched. After extrusion, the casing is inflated with air to expand the diameter to about 20 mm.

Strength properties are not significantly altered but the thickness of the casing is substantially less and vertical stretch is greatly reduced. Permeability studies show better permeability at the reduced thickness.

TABLE 1 Longitudinal Stretch After Regeneration −2.5%/STD −2.5%/STD 10% 10% 15% 15% 20% 20% 30% 30% Code 24 24 24 24 24 24 24 24 24 24 Stretch After Coug −2.5% −2.5% +10.0% +10.0% +15% +15% +20.0% +20.0% +30.0% +30.0% and Maintuined in Dryer Coug Temperature 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 42.5 Acid Conc. 132 132 132 132 132 132 132 132 132 132 Salt Conc 253 253 253 253 253 253 253 253 253 253 BDG 19 19 16.9 16.9 16.1 16.1 15.4 15.4 14.3 14.3 DFW 24 guage 24 guage 24 guage 24 guage 24 guage 24 guage 24 guage 24 guage 24 guage 24 guage Glye Tank Passes 2 2 2 2 2 2 2 2 2 2 Profile Time PIV rpm 73.5 73.3 Casing Analyses BDG 19.2 19.1 17 17.1 16.2 16.3 15.8 15.6 14.5 14.7 % BDG Change from 1.1 0.5 −10.5 −10.0 −14.7 −14.2 −16.8 −17.9 −23.7 −22.6 STD 19.0 pH 9.2 9.3 9.3 9.3 9.2 9 9.1 9.1 9.3 9.2 % Glycerine 15.7 15.6 15.7 15.7 15.3 15.1 15.7 15.7 15.9 15.6 Rewet Burst Press 33.84 34.01 33.04 33.92 33.84 33.69 32.84 31.93 30.32 31.20 Burst Dia. 38.24 35.72 38.33 37.96 37.02 37.79 38.37 35.05 35.51 35.67 Rewet Long. Instrons Film Thickness (inches) 0.0017 0.0016 0.0016 0.0015 0.0015 0.0014 0.0014 0.0014 Force to Break 1″ (lbs) 6.040 6.290 5.74 5.7 5.94 6.13 6.59 6.12 Max. Mod. (psi) 14519 15733 23811 25394 30164 33830 39478 38551 Max. Tensile (psi) 7102 7861 7172 7617 7925 8758 9416 8743 Rewet Trans. Instruns Film Thickness (inches) 0.0017 0.0016 0.0016 0.0015 0.0015 0.0014 0.0014 0.0014 Force to Break 1″ (lbs) 10.170 10.660 9.73 9.73 9.29 8.5 7.72 8.7 Max. Mod. (psi) 8831 9531 8982 9642 9276 9791 9370 9493 Max. Tensile (psi) 6007 6661 6086 6503 6195 6092 5516 6212

TABLE 2 STRETCH BURST EX- DRY (%) AFTER EX- DRY TRANS- BASIS THICK- REWET BURST DIA- TEST TRUDED END REGEN- TRUDED END VERSE WT NESS LENGTH PRESSURE METER PART CODE CODE ERATION FW (mm) FW (mm) STRETCH (%) (G/M 2) (MM) CHANGE (%) (CM HG) (mm) A 24 19 +40% 33.7 27 −20 25.4 0.023 −4.8 29.5 39 B 24 25 +40% 33.7 33.7 0 20.3 0.017 −6.2 28.4 38.6 C 24 30 +40% 33.7 40.5 +20 16.9 0.014 −6.4 28.9 41.8 STD 24 24 STD(−2.5) 33.7 32.3 −4.2 29.4 0.025 −0.2 30.34 43.4 STD 30 30 STD(−2.5) 44   41 −7 31.0 0.025 −0.5 25.7 40.4 

What is claimed is:
 1. A tubular film comprising a material which has been regenerated from a solution of cellulose and dried, said tubular film, after regeneration, being longitudinally stretched by from 10 to 40 percent and simultaneously transversely stretched by from −20 to +20 percent while wet, said stretch being maintained during drying, the combined longitudinal and transverse stretch being sufficient to increase the surface area of the film by at least ten percent from the surface area immediately after regeneration, said tubular film having a wall thickness of from about 60 to about 90 percent of the same tubular film without said combined stretch after regeneration and a longitudinal tensile strength greater than the same tubular film of the same thickness except that it is longitudinally stretched by less than 10 percent, said tubular film having a rewet longitudinal tensile strength greater than 6000 psi.
 2. The tubular film of claim 1 wherein said film is longitudinally stretched by from 10 to 20 percent.
 3. The tubular film of claim 1 having a wall thickness of from 18 to 25 microns.
 4. The tubular film of claim 3 having a wall thickness of from 21 to 23 microns.
 5. The tubular film of claim 1 having a permeability which is 20 to 50 percent greater than the same film without said combined stretch.
 6. The tubular film of claim 1 wherein the film has a burst pressure of at least 88 percent of the burst pressure of the same film without said combined stretch.
 7. The tubular film of claim 1 wherein the tube has a longitudinal shrinkage when rewet of at least 3 percent.
 8. The tubular film of claim 1 wherein the solution of cellulose is xanthate viscose.
 9. A tubular film which has been precipitated from a solution of cellulose in amine oxide and water and dried in a drier, said tubular film being precipitated and then longitudinally stretched by from 10 to 30 percent and simultaneously transversely stretched by from −20 to +20 percent while wet, the stretch being maintained in the dryer, the combined longitudinal and transverse stretch of said tubular film being sufficient to have a wall thickness of from 75 to 90 percent of an identical tubular film without said combined stretch after precipitation and a longitudinal tensile strength greater than the same tubular film of the same thickness except that it is longitudinally stretched by less than 10 percent, said tubular film having a rewet longitudinal tensile strength greater than 6000 psi. 